In a turbocharger engine, a turbocharger configured to supercharge intake air by using exhaust energy of an engine is mounted adjacent to a side wall of an engine body. An exhaust passage and an intake passage are formed in a housing of the turbocharger. A turbine chamber configured to accommodate a turbine communicates with the exhaust passage. A compressor chamber configured to accommodate a compressor impeller communicates with the intake passage. Exhaust air is supplied to the exhaust passage from the engine body, and intake air to be supplied to the engine body flows through the intake passage. The turbine is rotated around a turbine shaft by exhaust air to rotate the compressor impeller in the compressor chamber, which is connected to the turbine shaft to supercharge intake air.
Conventionally, there is known a turbocharger in which two independent turbo units are disposed in series in an exhaust passage. For instance, U.S. Pat. No. 8,720,421 discloses a dual stage turbocharger provided with a large turbo unit configured to be operated in all speed ranges of an engine and a small turbo unit configured to be mainly operated in a low speed range of the engine. Each of the large turbo unit and the small turbo unit includes a turbine chamber, a compressor chamber, and a turbine shaft extending between the turbine chamber and the compressor chamber.
In a turbocharger, there are always two demands i.e. a demand for increasing an output by efficiently transmitting kinetic energy from exhaust air to a turbine, and a demand for miniaturization. In the aforementioned dual stage turbocharger, the structure of an exhaust passage, particularly, the structure of an intra-turbine exhaust passage communicating between two turbine chambers tends to be complicated. This may cause a resistance against a flow of exhaust air in the exhaust passage, and kinetic energy for driving a turbine may be lost. Therefore, satisfying the aforementioned two demands at a high level is a challenging task.